FIG. 1 is an excitation circuit of a conventional excimer laser shown in, for example, page 5 of Denki Gakkai (Institute of Electrical Engineers) Technical Report (Part II) ("State of the art of short wavelength laser"). In this figure, numerals (1) and (2) are a pair of main discharge electrodes which are facing to each other; numeral (3) is a peaking capacitor attached in parallel with the main discharge electrodes (1) and (2); numeral (4) is a pulse generation capacitor, and one electrode thereof is connected to the main discharge electrode (1). Numeral (5) is a switch connected across the other end of the pulse generation capacitor (4) and the main discharge electrode (2); and in this prior art, it is comprised of a thyratron. Numeral (6) is a charging reactor, and numeral (7) is a charging terminal.
Next, the operation is elucidated. A positive high voltage is applied to the charging terminal (7), and the pulse generation capacitor (4) is charged up through the charging reactor (6). Graphs which illustrate temporal variations of voltages appearing on both ends of the pulse generation capacitor (4) and of the peaking capacitor (3), after closing the switch (5) at t=t.sub.0, are shown in FIG. 2. Electric charge stored in the pulse generation capacitor (4) during a period of t.sub.0 .ltoreq.t.ltoreq.t.sub.1 is transferred to the peaking capacitor (3), and at t=t.sub.1 a discharge starts between the main electrodes (1) and (2). In excimer lasers, although a preionization discharge prior to the main discharge is required, electrodes and its associating circuit for this are omitted in the elucidation in FIG. 1 and other figures for the present patent. During a period of t.sub.1 .ltoreq.t.ltoreq.t.sub.2, energy is injected into a main discharge taking place across the main discharge electrodes (1) and (2) from the peaking capacitor (3), thereby the laser oscillates. In a laser such as an excimer laser in which its discharging resistance is small (for example, 0.2 .OMEGA.), voltage appearing across both ends of the peaking capacitor (3) becomes an oscillatory waveform, and hence a reverse polarity voltage appears (FIG. 2, t=t.sub.2). At a time point when the oscillation almost terminates, a voltage (denoted by Vr in FIG. 2(a)) which is reverse in polarity with respect to that at a time of charging up appears across both ends of the pulse generation capacitor (4).
Another example of the conventional pulse laser excitation circuit is shown in FIG. 3. In this example, electric charge of a pulse generation capacitor (4) is transferred to a peaking capacitor (3) through a saturable reactor (8). This is a circuit referred to as an MPC circuit, in which, in order to reduce the loss in a thyratron switch (5), another pulse generation capacitor (9) and a current suppression reactor (10) are provided. The working waveform of this circuit is shown in FIG. 4, wherein similarly as in the conventional example previously described, after the discharge started across main electrodes (1) and (2) at t=t.sub.1, the current oscillates, and finally a voltage Vr which is reverse in the polarity with respect to that at the beginning appears across both ends of the pulse generation capacitor (4).
Since the conventional pulse laser excitation circuit is constituted as has been described above, a reverse polarity voltage appears across the both ends of the pulse generation capacitor; and this energy (=1/2cVr.sup.2) is dissipated as an arc or a streamer at the main discharge electrodes considerably later (for example, an order of 1 .mu.s later) after the main discharge appeared. This is a so-called after-current, which brings about a problem that it does not contribute to the laser generation, but damages the main discharge electrodes and shortens the lifetime of the electrodes. And, there is such a problem that, owing to a flow of after-currents, a high-repetition rate oscillation becomes impossible.